In a cellular wireless communication network, a representative of which is a portable telephone, a wide service area is constituted by dispersing a plurality of wireless base stations. Each wireless base station forms a wireless cell being a range in which devices can communicate with that wireless base station, and about one to six wireless cells are managed by a single wireless base station. In addition, by an overlap of the coverage of adjacent wireless cells (overlap region), even when a wireless terminal (User Equipment (UE)) crosses over the wireless cells, such a wireless terminal can continue communication by means of handover processing (Handover (HO)).
Normally, a wireless terminal measures the wireless quality (hereinafter referred to as “wireless quality of wireless cell”) of a radio wave (signal) arriving from each wireless cell, and connects to a wireless cell having the best wireless quality. An example of the wireless quality here is a received power or a received quality. The received power represents a receiving strength of a pilot signal or a reference signal transmitted from a wireless cell, and includes CPICH RSCP (Common Pilot Channel Received Signal Code Power) for the WCDMA (Wideband Code Division Multiple Access), (WCDMA is a registered trademark) network and RSRP (Reference Signal Received Power) for the LTE (Long Term Evolution) network. On the other hand, the received quality is a signal to noise ratio of a signal received from a certain wireless cell, and includes SINR (Signal to Interference plus Noise Ratio), CPICH Ec/No for the WCDMA (registered trademark) network, and RSRQ (Reference Signal Received Quality) for the LTE network.
So as to enhance a communication capacity of a wireless communication network, when the imbalance of traffic load is great between adjacent wireless cells, load balancing is often performed by adjusting the coverage of the wireless cells. Specifically, the traffic load balancing is attempted by either reducing the coverage of the wireless cells having a high traffic load or enlarging the coverage of wireless cells having a low traffic load. Some examples of means for adjusting the coverage are wireless cells' transmit power, antenna tilt angle, and an offset parameter to the wireless quality.
Here, examples of an offset parameter to the wireless quality are CIO (Cell Individual Offset), Qoffset, Event A3 offset, HO Hysteresis. The values substituted into these offset parameters are referred to as “offset value.” These offset values are reported from the wireless cell to “the wireless terminal whose serving cell (the connected wireless cell) is that wireless cell” or to “the wireless terminal whose best cell (the wireless cell having the highest wireless quality) is that wireless cell,” and are used for cell selection in the wireless terminal. Specifically, when each wireless terminal, while in communication, determines a wireless cell to connect to by comparing the wireless quality of the wireless cell (serving cell) to which that wireless terminal is connected, to the wireless quality of a wireless cell (target cell) other than the serving cell, that wireless terminal adds the reported offset value to the wireless quality of the target cell. Alternatively, when each wireless terminal, while not in communication (in an idle state), determines a wireless cell to connect to by comparing the wireless quality of the best cell to the wireless quality of a wireless cell (target cell) other than the best cell, that wireless terminal adds the reported offset value to the wireless quality of the target cell.
It should be noted that there are two types of offset parameters. One type of offset parameter uses a common value to all the target cells (hereinafter referred to as “offset parameter common to the target cells”); and the other type of offset parameter uses a separate value to each target cell (hereinafter referred to as “offset parameter specific to each target cell”). The above-described Event A3 offset, HO Hysteresis are offset parameters common to the target cells, and CIO, Qoffset are offset parameters specific to each target cell. For example, consider a case in which the CIO from the wireless cell A to the wireless cell B is set to be a negative value and the CIO from the wireless cell A to the wireless cell C is set to be a positive value. In such a case, “the wireless quality of the wireless cell B” measured by “the wireless terminal connected to the wireless cell A” is treated as a value lower from the actually measured value by the offset value, and as a result, the coverage of the wireless cell B with respect to the wireless A becomes smaller. Likewise, “the wireless quality of the wireless cell C” measured by “the wireless terminal connected to the wireless cell A” is treated as a value higher than the actually measured value by the offset value, and a result, the coverage of the wireless cell C with respect to the wireless cell A becomes larger. It should be noted here that there is also a method to add an offset value only to the wireless quality of the serving cell or to the best cell, without adding the offset value to the wireless quality of the target cell. However, in the present invention, such an offset parameter is treated as one type of offset parameter common to the target cells.
The technique to increase or decrease the coverage of the wireless cell taking advantage of the offset parameters with respect to the wireless quality as explained above is also known as a technique “Cell Range Expansion (CRE)”. Using a CRE enables each wireless cell to form coverage equal to or greater than in the case in which the largest transmission power is set. In the following explanation, the offset parameter with respect to the wireless quality is abbreviated as “wireless quality offset parameter” where necessary.
In the explanations hereafter, such an expression as “wireless terminal in the wireless cell” or “wireless terminal within the wireless cell” means that a wireless terminal whose serving cell or best cell is that wireless cell.
An operation to manually adjust the coverage of a wireless cell is a cause for increase in operation cost of the wireless communication network. With this in view, a technology to automatically perform such an operation has been examined. Some examples of it are CCO (Coverage and Capacity Optimization) and MLB (Mobility Load Balancing) which are use cases of SON (Self Organizing Network) under standardization in the 3GPP (3rd Generation Partnership Project).
Patent Literature No. 1 and Patent Literature No. 2 disclose methods to automatically optimize a wireless quality offset parameter. According to the method disclosed in Patent Literature No. 1, the traffic load of the wireless cell A is measured, and when the traffic load for the wireless cell A is high, the wireless cell B which is around the wireless cell A and has a large overlap with the wireless cell A is selected from the wireless cells around the wireless cell A and having a low traffic load. Then, the offset value from the wireless cell A with respect to the wireless cell B is increased, to enlarge the coverage of the wireless cell B with respect to the wireless cell A. On the other hand, according to the method disclosed in Patent Literature No. 2, the wireless base station NB1 transmits, to the other wireless base station NB2, a representative throughput value TP1 of the wireless terminal connected to itself. The wireless base station NB2 compares the received representative throughput value TP1 with the representative throughput value TP2 of the wireless terminal connected to itself. Then, the wireless quality offset parameter is controlled so as to correct the difference in representative throughput value (TP1−TP2) between the wireless base stations.